The outbreak, sparked by an infected visitor to Disneyland just before Christmas, spread not unlike a wildfire, infectious disease experts said. The virus jumped among clusters of vulnerable individuals up and down the state, where a large store of unvaccinated adults and children has been building up over the past two decades.

And, without substantial improvement in vaccination rates statewide, communities can expect more of the same for years to come, public health and infectious disease experts say.

“This stuff shouldn’t happen. This is clearly failure to vaccinate,” said Dr. George Rutherford, director of San Francisco’s Institute for Global Health and former state epidemiologist. “When the virus gets introduced to a community, it will find people who are susceptible. Now there’s a sufficiently large susceptible population to sustain transmission.”

Perhaps the most striking evidence of how the state’s measles landscape has changed is apparent in two previous Disneyland outbreaks, one in 1982 and another in 2001. Both started the same way the current outbreak likely did — someone from another country where measles is endemic visited the theme park while contagious. But there the similarities end.

The 1982 outbreak resulted in 14 cases and the 2001 outbreak had just five cases. This year, California has had 132 cases of measles as of Friday, 80 of them almost definitely linked to the theme park, and most of the others likely connected, too.

“I remember when we looked up that (1982) report, we thought, ‘Wow, if we make it to 14 cases that would be pretty impressive,’ ” said Jennifer Zipprich, lead epidemiologist investigating the current outbreak with the California Department of Public Health. “I would say this is quite unusual. The number of cases from the initial exposure and how it spread into the community is remarkable.”

For the majority of cases in the current outbreak, public health officials don’t know whether the person was vaccinated, usually because the individual didn’t know his or her immunization status. So no one can say with absolute certainty that Californians who have not been immunized are entirely to blame.

But unvaccinated individuals make up the bulk of cases — 75 percent — for which vaccine information was available, according to the state public health department. And the percentage of children who aren’t being vaccinated by choice, or by their parents’ choice, has been growing.

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